1. Field of the Invention
The present invention relates to improvements in multi-carrier code division multiple access (MC-CDMA) communication systems, and in particular to an improved method of transmitting and receiving data, wherein data bits or modulation symbols are spread over multiple sub-carriers in accordance with a code of predetermined length.
2. Description of the Related Art
In wireless communication systems, a series of transmitted signals are replicated and received at the receiver. These signals have different delays, amplitudes and phases along various paths due to the neighbouring environment. The superposition of these received signals causes amplitude and phase variations. This is called multi-path fading, and causes signal degradation and inter-symbol interference due to the delayed signals overlapping with following symbols.
In OFDM systems different data symbols are transmitted over different sub-carriers with symbol durations longer than the original symbol due to the serial to parallel operation. The degree of inter-symbol interference caused by multi-path fading is reduced as the symbol duration increases. In addition, an added cyclic prefix in OFDM symbols enables the receiver to remove inter-symbol interference and recover the orthogonality of sub-carrier signals.
An alternative method of reducing multi-path distortion is to use a multiple number of frequency carriers, or multi-carriers (MC), to send each data bit or symbol, thus providing an increased chance that data within one of the sub carriers is successfully received.
However, in the above communication systems, if other users apply the same method, distortion can result due to the interference of two different signals being transmitted at the same sub frequency. This has resulted in the development of Multi-Carrier Coded Division Multiple Access (MC-CDMA).
In MC-CDMA, a unique code is allocated to each user of the system. The code is orthogonal, or non-overlapping, so that each code does not interfere with any other code. The code is applied to each data bit or symbol at the transmitter end in order to spread it over the useable frequency bandwidth. In MC-CDMA systems, the spreading factor is the number of frequency sub-carriers over which a data signal is spread. Therefore, the length of the code determines the spreading factor, or processing gain.
FIG. 1 shows a known MC-CDMA transmitter block diagram where the length of the spreading code (processing gain) is equivalent to the number of useable frequency sub-carriers. A data stream 101 for one user is indicated. The data stream comprises data symbols 103, and is applied to the input of a de-multiplexer 105. Each symbol 103 has a pre-determined orthogonal code 109 applied to it 107 in order to spread the data symbol 103 over the available transmission bandwidth. After a pre-determined number of symbols, which is equivalent to the number of useable frequency sub-carriers, have been spread by a unique spreading code, they are summed together by the summing circuitry 115. The output of the summer 115 is converted from serial to parallel using the serial to parallel converter 117. The outputs of the serial to parallel converter 117 are applied to the input of an Inverse Fast Fourier Transform (IFFT) processor 119. The frequency bandwidth over which the data symbols have been spread is indicated by 113 in FIG. 1. The IFFT converts the frequency domain signal into a time domain signal, which is then converted from a parallel to serial signal using the parallel to serial converter 121. The transmitter aerial 123 then transmits the signal.
Guard bands are provided in the system to minimise interference of the transmission frequency range with other frequency ranges that are allocated for other purposes. Therefore, the input of the IFFT 119 has null points 118 set at the top and bottom of the available frequency bandwidth being used. The null area is usually set at 25% of the total available bandwidth, with 12.5% at the top of the frequency range and 12.5% at the bottom of the frequency range.
The example shown in FIG. 1 corresponds to the case where the spreading factor is equivalent to the number of useable frequency sub-carriers. However, if there are a large number of frequency sub-carriers, this can be unrealistic because the spreading factor is proportional to the receiver complexity. In order to reduce the complexity of the receiver, additional de-multiplexing of the data stream being transmitted is required to reduce spreading factor.
De-multiplexing is also required to match the number of useable frequency sub-carriers to an integer multiple of the spreading factor. The length of the IFFT is equal to the number of total sub carriers, and is a number equal to 2m, where m is an integer. The spreading factor is a number equal to 2n, where n is an integer. The number of useable frequency sub-carriers is equal to 75% of the IFFT length due to the null points in the guard bands. A problem therefore arises, as 75% of 2m does not fit to the spreading factor 2n.
An example is given taking the case where the IFFT length is set at 1024 (210). This means the number of useable sub-carriers is equal to 75% of this length, i.e. 768. As the spreading factor is a number equal to 2n, where n is an integer, in order to provide orthogonality the spreading factor needs to be decreased to a value at least equal to 512, and so a third of the useable sub-carriers are not used. In order to make full use of the useable frequency sub-carriers, data symbols need to be de-multiplexed appropriately.
The same problems are associated with the FFT inputs at the transmitter end of the system.